Once you placed the usrcheat.dat file in the __rpg/cheats/ folder, then you can access the cheat list by Highlighting a game and pressing Y button (or Start menu>File information), then select X button to access the cheat list.

Like Tranfeer said, you need to enable cheat in game first :
Start menu > Settings > Press R button to go to the Patches screen and set "Cheat in Game (AR)" to ON.